The drill bit at the lower end of a drilling string is normally rotated by multistage turbines driven by liquid, normally drilling mud, that is pumped at high pressure and in large volumes down through the string. As the output speed of such turbines is rather large, it is necessary to provide a stepdown transmission between the turbine output and the drill bit. Furthermore it is necessary to provide a passage so that the drilling mud, after traversing and driving the turbines, can exit from the lower end of the drill string at the bit and thence pass upwardly around the drill string, carrying with it the chips and particles created by the drill bit.
The amount of space is greatly limited, because on the one hand the maximum outer diameter of the drill string will be slightly smaller than the inside diameter of the bore so that the drilling mud can move upwardly around the string along the invariably nonrotating outer tube of the string, and on the other hand because the central axial passage through which the drilling mud passes downwardly in the string must be as large as possible so that large volumes of mud at high pressure can be supplied to the bottom end of the drill string, both to power the drive turbines and ensure good drilling.
Transmissions are known using planetary gearing that is capable of fitting within the relatively small available space. As a result of the small amount of space, however, the ratio of teeth between the gears lies between 2.2:1 and 1.7:1 so that the maximum transmission stepdown ratio lies somewhere between 1:1 and 3.2:1. Since the service life of the drilling head is dependent in large part on the rotation speed, rotating it at high speed wears it out rapidly. Furthermore the passage through the center of such a drill string is relatively limited in cross-sectional size so that often an inadequate amount of liquid for cooling and clearing of the bit is passed through the drill string. The planetary gearing often wears out relatively rapidly due to the extremely large forces that are applied to the various bearings. The gears themselves often wear out prematurely as a result of transmitting extremely great loads. In fact in these known transmissions the gear load is largely proportional to the load and to the spacing of the planet axes from the main axis of the gearing as well as the number of planet gears. Obviously in a limited space these factors are also greatly limited.